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In arthropods, the intracellular bacteria Wolbachia often induce cytoplasmic incompatibility ( CI) between sperm and egg, which causes conditional embryonic death and promotes the spatial spread of Wolbachia infections into host populations. The ability of Wolbachia to spread in natural populations through CI has attracted attention for using these bacteria in vector-borne disease control. The dynamics of incompatible Wolbachia infections have been deeply investigated theoretically, whereas in natural populations, there are only few examples described, especially among incompatible infected hosts. Here, we have surveyed the distribution of two molecular Wolbachia strains ( w Pip11 and w Pip31) infecting the mosquito Culex pipiens in Tunisia. We delineated a clear spatial structure of both infections, with a sharp contact zone separating their distribution areas. Crossing experiments with isofemale lines from different localities showed three crossing types: w Pip11-infected males always sterilize w Pip31-infected females; however, while most w Pip31-infected males were compatible with w Pip11-infected females, a few completely sterilize them. The w Pip11 strain was thus expected to spread, but temporal dynamics over 7 years of monitoring shows the stability of the contact zone. We examined which factors may contribute to the observed stability, both theoretically and empirically. Population cage experiments, field samples and modelling did not support significant impacts of local adaptation or assortative mating on the stability of w Pip infection structure. By contrast, low dispersal probability and metapopulation dynamics in the host Cx. pipiens probably play major roles. This study highlights the need of understanding CI dynamics in natural populations to design effective and sustainable Wolbachia-based control strategies. [ABSTRACT FROM AUTHOR]

Wolbachia are maternally inherited endosymbionts that can invade arthropod populations through manipulation of their reproduction. In mosquitoes, Wolbachia induce embryonic death, known as cytoplasmic incompatibility (CI), whenever infected males mate with females either uninfected or infected with an incompatible strain. Although genetic determinants of CI are unknown, a functional model involving the so-called mod and resc factors has been proposed. Natural populations of Culex pipiens mosquito display a complex CI relationship pattern associated with the highest Wolbachia ( wPip) genetic polymorphism reported so far. We show here that C. pipiens populations from La Réunion, a geographically isolated island in the southwest of the Indian Ocean, are infected with genetically closely related wPip strains. Crossing experiments reveal that these Wolbachia are all mutually compatible. However, crosses with genetically more distant wPip strains indicate that Wolbachia strains from La Réunion belong to at least five distinct incompatibility groups (or crossing types). These incompatibility properties which are strictly independent from the nuclear background, formally establish that in C. pipiens, CI is controlled by several Wolbachia mod/ resc factors. [ABSTRACT FROM AUTHOR]

Wolbachiais a group of maternally inherited endosymbiotic bacteria that infect and induce cytoplasmic incompatibility (CI) in a wide range of arthropods. In contrast to other species, the mosquitoCulex pipiensdisplays an extremely high number of CI types suggesting differential infection by multipleWolbachiastrains. Attempts so far failed to detectWolbachiapolymorphism that might explain this high level of CI diversity found inC. pipienspopulations. Here, we establish thatWolbachiainfection is near to or at fixation in worldwide populations of theC. pipienscomplex.Wolbachiapolymorphism was addressed by sequence analysis of theTr1gene, a unique transposable element of the IS5 family, which allowed the identification of fiveC. pipiens Wolbachiastrains, differing either by nucleotide substitution, presence or absence pattern, or insertion site. Sequence analysis also showed that recombination, transposition and superinfection occurred at very low frequencies. Analysis of the geographical distributions of eachWolbachiastrain amongC. pipienspopulations indicated a strong worldwide differentiation independent from mosquito subspecies type, except in the UK. The availability of this polymorphic marker now opens the way to investigate evolution ofWolbachiapopulations and CI dynamics, in particular in regions where multiple crossing types coexist amongC. pipienspopulations. [ABSTRACT FROM AUTHOR]